On-line measurement of the level of liquids or powders in storage tanks is extensively used for process control in the chemical, petrochemical and medical industry, as well as in custody transfer, marine and transportation operations. For this purpose, there are a variety of mechanical, electrical, magnetic methods and instruments (transmitters) for detecting point and continuous level for liquids and powders. For example, there are continuous level transmitters including non-contact radar, guided wave radar (GWR), non-contact ultrasonics, differential pressure, capacitive, magnetostrictive, displacers, and laser.
Continuous level measurements can be made either by a contact method (e.g. GWR, differential pressure, capacitive, displacers), where the transmitter is immersed in the liquid, or by keeping the transmitter outside the liquid (non-contact method), such as in the case of non-contact radar, ultrasonics, and laser. Each of these methods has both advantages and limitations, and in practice, the selection of the most appropriate method for a particular situation is made by taking into consideration the specific application requirements, such as the chemical nature of liquid, process conditions, etc. For many applications where an actuation decision is taken when the liquid (or powder) level is too high or too low, point level measurement by using switches is generally used, such as using a vibrating fork, capacitive, float and displacer switches.
A more complex problem is the detection of both the level and interface between two immiscible liquids, such as in the case of oil and water, where oil having lower gravimetric density will remain on the upper side of the interface, while water having a higher gravimetric density will occupy the bottom side of the interface. For such a case, contact methods are generally used (e.g., GWR, differential pressure), where both the hardware and software algorithms are more complex (compared to non-contact methods) such as in the case of differential pressure method, where two pressure sensors are used, or complex software is used, in order to discriminate between upper level, the interface, the bottom side or parasitic reflections, such as in the case of the GWR method. In addition, to detect an interface between two liquids, a number of conditions need to be fulfilled, including the dielectric constant difference between the two media should generally be at least equal to 10. Detection of more than one interface is even more challenging, if not impossible, for known contact methods.